[RE: VCO]

[Mikko Helin]

Thanks for all responses, Martin & Harry. It seems that there are
minor problems with this circuit, which was expected. It seems
(from datasheets) that the threshold voltages really may vary a lot.

However, there's another chip that can be used for the circuit.
It's the NE555 timer chip. Just connect the trigger input
(the low level threshold comparator input) and the threshold
comparator input (the high level comparator input) pins together
(pins 2 and 6) and cap from this point to ground. Then connect
the output pin (pin 3) via a resistor and diode to the previous
point. Then connect the same point to exponential converter
which current source sinks current (like in ASM-1 or Terry Michaels
saw relaxation oscillator in EN). The two comparators compare 
input levels to 1/3 and 2/3 Vcc voltages, and set and reset 
the flip-flop inside accordingly. Now the output level
from the cap is between +5 V and +10 V with +15V source, and
as there's the separate control voltage input pin, you could
feed stabilized voltage ref directly into that pin. 
As the 555 spikes a lot when fired - like the BAT 555's, but
with less smoke (though I don't use to smoke at least when 
I'm not drinking) -  it would be nice to have larger caps on 
Vcc and small resistors to PSU or separate 78L15 for each 555 to
avoid sync through PSU lines if multiple 555 VCO's are used 
(like in Fatman).

Btw. Are there any particular reasons why 555's should be avoided
(other than lung cancer)?

-Mikko

>
>
>> Some addl thoughts... the ESD diodes shouldn't be a player in the inaccuracy
>> of the circuit because the operating point will be far away from the point
>> where they would be biased even a little bit. The circuit will run at 1/2 Vcc
>> plus or minus the hysteresis band. The very limited amplitude should keep
>> these errors almost constant. That doesn't mean it works, it means this isn't
>> the biggest problem. Can anyone think of a use for a sawtooth wave where we
>> have no control over amplitude, bot can control frequency. It's no sweep
>> generator... ;-) Harry Bissell
>
>NO!
>
>The junctions are not forward biased , but there is diode leakage current.
>These diodes are parasitic elements of the CMOS process, the technology is
>not designed to do very low leakage diodes, just NMOST & PMOST.
>Leakage rises exponentially with temperature. @-40C these parts may have
>~1nA input leakage, @120C 1uA (mil grade).
>
>> 
>> The problem is even worse than diode leakage. C-MOS schmitt-trigger
>> threshholds vary with manufacturer and aren't guranteed. Sawtooth amplitude
>> is proportional to the hysteresis. And the C-MOS gate has a very limited
>> ability to recharge the cap. especially with the series resistor and diode.
>
>As long as the threshold margins are constant, I don't care about
>amplitude.  The schmitt's are quite fast, you have to pay for that with
>temp and supply variation (via supply you could adjust the amplitude).
>The temp error can be a problem for large temp variations (thermostat ?).
>
>If the drive capabilty is nut sufficient one could add a switching tranny
>instead of diode, like I proposed.
>
>m.c.
>